Huntington's disease (HD) is the most common adult onset neurodegenerative disease, and it is caused by a polyglutamine (polyQ) expansion in the huntingtin (htt) protein. Htt is present in almost every cell type, but dopamine receptor 2 (D2R)-containing medium spiny striatal neurons (D2R-MSSNs) are among the first to die. We hypothesize that at least three factors conspire to render certain neurons susceptible to degeneration in HD. First, we hypothesize that cell autonomous mechanisms govern how cells recognize and adapt to misfolded proteins. In the previous funding period, we developed a robotic microscope system to determine the prognostic value of cellular changes for HD. We discovered that neurons that form inclusion bodies (IBs) live longer than those that do not, and that diffuse forms of mutant htt predict death. Since then, we have obtained preliminary data showing that cortical neurons, which survive mutant htt better than striatal neurons, form IBs more readily and at lower doses of mutant htt. We will use our HD model and robotic microscopy in Specific Aim 1 to determine if differences in htt turnover and IB formation explain increased susceptibility to htt-induced degeneration of striatal versus cortical neurons. Second, we posit that the vulnerability of D2R-MSSNs to mutant htt may be linked, in part, to their very dense dopaminergic and glutamatergic innervation. Mutant htt increases excitotoxic signaling by glutamatesensitive A/-methyl-D-aspartate (NMDA) receptors, and our preliminary data show that dopamine potentiates htt-induced IB formation and striatal neuron death, which can be reduced with a D2R-specific antagonist. In Specific Aim 2, we will assess cell-autonomous and non-autonomous contributions to the susceptibility of D1AR- versus D2R-MSSNs to mutant htt, using robotic microscopy, transgenic striatal neurons that express GFP in D1AR or D2R-positive cells, and manipulations of glutamate and dopamine signaling. Third, we hypothesize that the need of the striatum for brain-derived neurotrophic (BDNF) factor support from the cortex, and the discovery that mutant htt reduces BDNF transcription in the cortex, also contributes to MSSN susceptibility in HD. Among other things, BDNF activates Akt. Our preliminary data show that Akt is highly neuroprotective independent of htt, and that it can phosphorylate htt and reduce polyQ-dependent toxicity. We will examine the contribution of these pathways in Specific Aim 3 by determining in vivo if the Akt phosphorylation sites in htt regulate polyQ-dependent toxicity and if BDNF can rescue httinduced striatal neurodegeneration.